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Chapter: Object Oriented Programming(OOP) : Overview of Java

Java - Classes and Objects,Methods

A Java program consists of one or more classes



n A Java program consists of one or more classes

n A class is an abstract description of objects

n Here is an example class:


 class Dog { ...description of a dog goes here... }

Here are some objects of that class:

       Here is another example of a class:


       class Window { ... }


       Here are some examples of Windows:



1. Classes contain data definitions

n    Classes describe the data held by each of its objects


n    Example:


n class Dog { String name; int age;


...rest of the class...



n    A class may describe any number of objects


n    Examples: "Fido", 3;  "Rover", 5; "Spot", 3;


n    A class may describe a single object, or even no objects at all


2. Classes contain methods

n    A class may contain methods that describe the behavior of objects


n    Example:


n    class Dog {




void bark() { System.out.println("Woof!");




n    When we ask a particular Dog to bark, it says “Woof!”


n    Only Dog objects can bark; the class Dog cannot bark


3. Methods contain statements

n    A statement causes the object to do something


n    (A better word would be “command”—but it isn’t)


n    Example:


n    System.out.println("Woof!");


n    This causes the particular Dog to “print” (actually, display on the screen) the characters Woof!


Methods may contain temporary data

n    Data described in a class exists in all objects of that class


n    Example: Every Dog has its own name and age


n    A method may contain local temporary data that exists only until the method finishes


n    Example:


n    void wakeTheNeighbors( ) {


int i = 50; // i is a temporary variable while (i > 0) {


bark( ); i = i – 1;





Classes always contain constructors

n    A constructor is a piece of code that “constructs,” or creates, a new object of that class


n    If you don’t write a constructor, Java defines one for you (behind the scenes)


n    You can write your own constructors


n    Example:


n    class Dog {


String name; int age;


Dog(String n, int age) { name = n;


this.age = age;






Diagram of program structure



class Dog { String name; int age;


Dog(String n, int age) { name = n;


this.age = age;



void bark() { System.out.println("Woof!");



  void wakeTheNeighbors( ) { int i = 50;


while (i > 0) { bark( );


i = i – 1;




public static void main(String[ ] args) { Dog fido = new Dog("Fido", 5);






  } // ends the class


Method Definitions


• Method definition format

return-value-type method-name( parameter-list )


declarations and  statements


–   Method-name: any valid identifier

–   Return-value-type: data type of the result

         void - method returns nothing

         Can return at most one value

–   Parameter-list: comma separated list, declares parameters

         Method call must have proper number and type of parameters

–   Declarations and statements: method body (block)

         Variables can be declared inside blocks (can be nested)

         Method cannot be defined inside another function

–   Program control

–   When method call encountered

         Control transferred from point of invocation to method

–   Returning control

         If nothing returned:  return;

–   Or until reaches right brace

         If value returned:  return expression;

–   Returns the value of expression

–   Example user-defined method:

Public int square(                   int y )


return y * y



         Calling methods

–   Three ways

         Method name and arguments

–   Can be used by methods of same class

–   square( 2 );

         Dot operator - used with objects

–   g.drawLine( x1, y1, x2, y2 );

         Dot operator - used with static methods of classes

–   Integer.parseInt( myString );

–   More Chapter 26

         More GUI components

–   Content Pane - on-screen display area



         Attach GUI components to it to be displayed

         Object of class Container (java.awt)

–   getContentPane

         Method inherited from JApplet

         Returns reference to Content Pane

Container c = getContentPane();

–   Container method add

         Attaches GUI components to content pane, so they can be displayed

         For now, only attach one component (occupies entire area)

         Later, learn how to add and layout multiple components

c.add( myTextArea );


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